The present invention relates to a hydro-pneumatic suspension system for use in a wheel or axle suspension of a motor vehicle, such as a farm or industrial vehicle.
Such a suspension system includes at last one hydraulic suspension cylinder connected between the vehicle body and the wheel and/or vehicle axle. The cylinder has a piston chamber and rod chamber, each connected to at least one accumulator. Valve systems can controllably connect the piston chamber and rod chamber to a pressure source and to a tank. Such a suspension system can be a single-wheel suspension or an axle suspension.
The design of a hydro-pneumatic suspension system is a particular challenge especially for farm or industrial equipment, such as tractors, because a wide range of wheel or axle loads are possible. The axle load range usually exceeds the pressure ratio possible with diaphragm/membrane type accumulators. In order to be able to adhere to the operating range of the accumulator, the system is therefore pre-loaded. This pre-load acts like an additional axle load, thus allowing a decrease in the ratio between minimal and maximum axle load.
The system is rarely pre-loaded by means of mechanical pre-stressing, during which process also a single-acting cylinder can be used. Generally a double-acting hydraulic cylinder is employed, which allows the rod chamber side to be pre-loaded to a specific pressure via an accumulator.
The pressure in the rod chamber can either correspond to the maximum system pressure as described in published German patent application DE 43 08 460, or the pressure can be adjusted by means of a pressure regulator to a specific pre-selected value as described in German patent DE 42 42 448. In both methods, as is the case with any hydro-pneumatic system, the spring rate essentially has a quadratic dependency on the axle load in accordance with a given function. For a consistent natural frequency of the tractor body mass in combination with the suspension system, however, a linear function would be ideal.
Published German patent applications DE 197 19 076 and DE 197 19 077 describe systems which include a hydraulically actuated pressure control valve in the line leading to the rod chamber, and the control valve spring regulating force is adjusted as a function of the pressure in the line leading to the piston chamber. Thus, the pressure in the rod chamber is supposed to be regulated as a function of the pressure in the piston chamber of the hydraulic cylinder, wherein the additional load in the rod chamber is greatest at low load levels and is reduced at higher load levels down to a minimum value. This is intended to improve, among other things, the driving comfort.
Published German patent application DE 41 20 758 describes a hydro-pneumatic vehicle suspension system of the above-mentioned kind in which, a hydraulically actuated control valve is arranged in the feed line to the rod chamber for the purpose of adjusting to a large range of axle load. The control valve is subjected to the pressure in the rod chamber and to the pressure of the piston chamber so that the pressure in the rod chamber is regulated as a function of the pressure in the piston chamber. This is intended to provide a greater stiffness spring in the smaller axle load range.
Different ballast conditions on the vehicle can lead to the same axle load. For example, in order to compensate the weight of a heavy plow and apply sufficient load onto the front axle, ballast weights can be mounted on the front of the tractor. This ballast balancing can result in the same axle load on the front axle, such as on a tractor that is equipped neither with a plow nor with front weights. In the known vehicle suspension systems, however, the rigidity cannot react to different ballast conditions and operating or driving states of the vehicle—the rigidity is not dependent on these conditions and solely depends on the respectively present axle load. This can result, when there is a lot of ballast, in a tractor suspension which is too soft with respect to the characteristic pitch frequency, and, when there is no ballast, in a tractor suspension which is too stiff.